This invention relates to an improvement in a traverse for a thickness gauge and more particularly to a nuclear gauge traverse drive that rapidly moves the gauge from measurement point to measurement point where the gauge resides for the considerable time necessary to measure thickness. Reducing the time that a traverse takes to make a single pass across continuously moving product being manufactured will improve system response time by enabling quicker response to tolerance departures in the measured parameter or trends toward out of spec product and hence improve the performance of any closed loop control used with the traverse.
An extruder makes plastic sheet by heating plastic in the manner well-known in the art and forcing it through a die that is in the form of a narrow slit. The thickness of the sheet at each point is determined by a number of factors such as the temperature of the die lips, the viscosity of the plastic at that point, surface finish on the die lips, etc., but it is also determined in large part by the spacing between the die lips at the point in question.
To have the thickness of the sheet as uniform as possible across the width is important in extruding sheet, otherwise subsequent use of that sheet will have highly variable results. No die can be made absolutely perfect, and to allow some means for adjusting the die opening at the various points across the width, it is customary to build slight flexibility into one of the lips and mount a series of bolts across the width so that the die lips may be flexed slightly to increase or decrease spacing between the lips at that point. Typical spacing of these die bolts along the width (i.e. the direction transverse to the direction of movement of the extruded product) is 1 1/8". There may be 67 die bolts across the width of a 75" die. Thirty inch (30") wide dies have 26 die bolts.
Nuclear thickness gauges are used to monitor the thickness of extruded sheet. To measure the thickness profile accurately across the width of the sheet, the gauge is mounted on a traverse that carries the gauge across the sheet. A monitoring device such as the Harrel CP-680 DIGIPANEL sheet thickness controller used with the gauge notes the thickness reading as the gauge passes points across the sheet width corresponding to each die bolt position and creates a profile display that is a histogram or bar chart of thickness in each die bolt position.
In a manually adjusted system the operator notes where the sheet thickness deviates from the norm and adjusts the appropriate bolt or bolts manually to eliminate the discrepancy. Harrel, Incorporated, the assignee of this invention, makes two types of automatic control systems which can adjust the thickness profile automatically. One uses the "AUTO-FLEX" die system available from Extrusion Dies, Incorporated. This die system uses die bolts made from a metal having a high co-efficient of thermal expansion. Changing the heat applied to the bolt by a heater wrapped around such a die bolt makes it possible to adjust the die lip spacing by remote control.
The second system, developed by the assignee Harrel, uses heaters along the die lips. The temperature of the die lip at each point also affects the amount of plastic which comes through at that given point, and hence the thickness at that point. A modern digital controller such as the Harrel CP-680 DIGIPANEL system can, by dictating the current to each heater, change the lip temperature at each heater location and accomplish the same result as if the spacing of the die lips had been changed. Here it is the location of the heaters that is of interest in measuring thickness at points across the sheet width. Sheet thickness should be measured at locations corresponding to the heaters. In any case, one is interested in 60 or so discrete locations in a 90" wide die, for example.
A thickness gauge that is used for thickness measurements of the type described is known as a gamma backscatter gauge. The Harrel TG-600 nuclear is such a gauge. This gauge requires an appreciable time to reach equilibrium. Time constants of 1 and 3 seconds are common. They might range as widely as 0.5 to 10 seconds. A time constant of 1 second, for example, means that if the plastic is thicker opposite a particular die bolt, one has to stay in the vicinity of the measurement point corresponding to that die bolt for at least 3 seconds to measure 99% of the discrepancy. This requirement sets the ultimate limit on how fast one can afford to run a traverse, which operates at constant speed. The gauge cannot go from one measurement point to the next in less than 5 seconds or so. With 60 bolts this means that the gauge only makes one scan and measures one thickness profile in about 300 seconds or 5 minutes, and even this is a compromise. Even the use of a constant speed traverse in a faster than ordinary case, say a gauge travelling slowly enough to be in proximity to each measurement point for just one time constant of 0.5 seconds will, when used with 20 or more measurement points result in a very slow source of control information by the standards of closed loop controls. Even that control would benefit appreciably from the connection described herein.